LibreOffice oss-fuzz, crashtesting, coverity Overview Oss-Fuzz - - PowerPoint PPT Presentation

LibreOffice oss-fuzz, crashtesting, coverity

Overview

• Oss-Fuzz
• Crashtesting
• Coverity

Oss-Fuzz

Overview

• Continuous Fuzzing of our import filters
• Thanks to Google we get to use their infrastructure and resources

Configuration

• Build remotely on google’s side

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Calls our bin/oss-fuzz-build.sh

• 45 fuzzer targets in vcl/workben
• Each one is built with

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libfuzzer + asan

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libfuzzer + ubsan

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afl + asan

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honggfuzz + asan => 180 total

Configuration

• No dynamic libraries allowed

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A serious pain for us

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distro-configs/LibreOfficeOssFuzz.conf

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Reuse --disable-dynamic-loading intended for iOS

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Individual fuzzers are unfortunately v. large

• Run without config layer

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Hardcoded suitable default for –enable-fuzzers

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utl::ConfigManager::IsAvoidConfig()

• https://dev-www.libreoffice.org/corpus/

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Contains our seed corpuses for 60 file formats

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15 are dtardons and co’s dlplib filters and are fuzzed separately

Oss-Fuzz Reports per Year

• Over 1100 issues over four years

2017 2018 2019 2020 50 100 150 200 250 300 350 400 450 500

• ss-fuzz: Last 4 Years

Projected Filed

Year Reports

• More than one a day in 2017 and 2018
• 113 this year to date, estimate 142 by end of year
• This years uptick due to a new route from sftfuzzer into old untested code

What a report looks like

Auto minimize

Sample ubsan bug

• Buggy change, the unsigned

short is promoted to int, undefined behavior in addition

• Change back to a larger

unsigned type

Timeouts

• Sometimes timeout is genuine infinite loop

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More often it’s just slow

• OssFuzz will report a maximum of one timeout per fuzzer
• Fix a timeout, another typically gets reported soon after
• Limit input size with a .options files

[libfuzzer] max_len = 65536

• Some file formats have ~infinite decompression support

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Tiny input can legitimately provide mega data to process

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Examine FUZZ_MAX_INPUT_LEN (from .options) at runtime and limit to some factor of that

OOM

• Limit memory usage with

setenv("JPEGMEM", "768M", 1); setenv("SC_MAX_MATRIX_ELEMENTS", "60000000", 1); setenv("SC_NO_THREADED_CALCULATION", "1", 1);

• Pre-allocating buffers depending on potentially lying headers

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Often a known relationship between remaining length of the file and the amount of data that it can produce

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So short reads can be predicted before buffer allocation

• GIF’s have a max compression of ~1:2560,

Current Open Bugs

• 10 open bugs

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All Timeouts

CrashTesting

Overview

• Document Corpus

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Most scraped out of various bugzilla instances with get-bugzilla-attachments-by-mimetype

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116,200 files

• Import them all

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With Markus Mohrhard’s test-bugzilla-files

• For many formats, then export to multiple formats
• Reimport exported output
• Report failed imports/exports
• Backtraces extracted from coredumps

New Setup

• New Hardware this year

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Next day results

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Vs ~3 days with old setup

• Thanks to Adfinis

12 Months of Importing

• Persistent <10 cluster of failures

1 4 7 1 1 3 1 6 1 9 2 2 2 5 2 8 3 1 3 4 3 7 4 4 3 4 6 4 9 5 2 5 5 5 8 6 1 6 4 6 7 7 7 3 10 20 30 40 50 60 70

Import Failures

Import

Build Failures

12 Months of Exporting

• Large jumps as regressions detected and fixed

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73 50 100 150 200 250 300

Export Failures

Export

Build Failures

Coverity

Configuration

• Build locally with coverity’s tooling
• Outputs a big blob which we upload to their server which does

the analysis

• https://scan.coverity.com/projects/libreoffice
• Project settings are open, no need to apply to be a “member” to

see the findings

• Contemporary coverity scans both C++ and Java
• Coverity currently supports C++17, but not C++2a

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patch configure.ac to not try c++2a for the coverity run

• We only scan LibreOffice, not dependencies

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distro-configs/LibreOfficeCoverity.conf

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So no ignored “external” category anymore

Example warning

• uninit_member

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If a class initializes none of its members in its ctors there is no warning as its assumed to be intentional

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If it initializes most of them, it warns about the uninitialized ones

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A common mistake is with a class with multiple ctors, new member gets added and initialized in one ctor but not the other

warning type

Pattern for waiving warnings

• An issue can be marked as a false positive or intentional in the web UI

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But that only affects that coverity instance. Red Hat runs another one f.e.

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If the code changes enough coverity will loose the ability to detect its the same code and reissue the warning

• INTENTIONAL pattern

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// coverity[WARNING] - OPTIONAL_COMMENT

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WARNING is the text before the : in the report

– – –

• FALSE POSITIVE pattern

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// coverity[WARNING : FALSE] - OPTIONAL_COMMENT

Pattern to indicate program exit

• // coverity[+kill] indicates that the annotated function is intended to

kill the program

– – –

• We use this in cppunit to indicate that that Asserter::fail is intended

to exit the program. In reality it throws a deliberately unhandled exception which would be warned about otherwise

• Note that –enable-assert-always-abort is active for our coverity

builds so failing asserts terminate program flow so coverity warnings about “impossible” situations are resolvable by adding appropriate asserts

Tainted data

• Coverity detects common byteswapping techniques as indicating

that data is probably untrusted tainted data

• Very helpful for our general file format parsing, but not for our own

legacy registry data format

• __coverity_tainted_data_sanitize__ can be used to sanitize the data

Tainted data

• Validating untrusted data

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A simple sanity test of tainted example

Outstanding vs Fixed defects

Gap between requiring C++17 and coverity support

Coverity Stats 2020